1. Technical Field
The present invention relates to the field of thin film write heads.
2. Background Art
Data is stored on magnetic media by writing on the magnetic media using a write head. Magnetic media can be formed in any number of ways, such as tape, floppy diskette, and hard disk. Writing involves storing a data bit by utilizing magnetic flux to set the magnetic moment of a particular area on the magnetic media. The state of the magnetic moment is later read, using a read head, to retrieve the stored information.
Data density is determined by the amount of data stored on an area of magnetic media and depends on how much area must be allocated to each bit. Data on magnetic media is often stored in a line or track. Magnetic media often have multiple tracks. In the case of the disk, the tracks are nested annular rings. More bits per ring and more rings per disk increases data density. Data density or areal density, therefore, is determined by both the bit length and by the width of the bit. To decrease bit size, head size is decreased by fabricating thin film read and write heads. Thin film heads commonly employ separate write and read heads.
Thin film write heads are typically formed by depositing and etching layers of magnetic, non-magnetic, dielectric, and electrically conductive materials to form the structures of the head, such as a core, a conductor winding, and upper and lower pole tips and yokes. Write heads typically do not contact the magnetic media but instead are separated from the magnetic media by a layer of air or air bearing. Magnetic flux generated between poles of the write head acts across the air bearing to change the magnetic moment of an area on the magnetic media.
Since 1991, the compound growth rate of areal density in hard disk drives has been an accelerated 60% per year. One of the major challenges for magnetic recording heads is to develop high moment writer materials. Higher moment writer materials can generate sufficiently high flux to write and overwrite media with high coercivity. High medium coercivity is essential for sustaining small magnetic bits consisting of narrow transitions.
In order for high moment material to be useful for write head applications, several requirements have to be met. The material must be magnetically soft with low coercivity value, and it also must have high permeability and low magnetostriction. As ever smaller structures must handle higher magnetic flux, the write head structures, and in particular the yoke and pole tips, become susceptible to saturation. As a result, it is necessary to form the yoke and pole structures of material with a sufficiently high magnetic moment to handle high flux density without saturating.
The rate or frequency that data is stored to the media is an important measure of the operational performance of the write head. One problem with operating at higher frequency is that the permeability of the material diminishes. As the magnetic flux changes, it generates a corresponding electrical field encircling the magnetic flux, opposing the change. In an electrically conducting material, the induced electrical field generates current, referred to as eddy current, which in turn generates an opposing magnetic field. This not only limits flux switching time, but also causes saturation near the edge of the structure, thus lowering the permeability of the structure at high frequency.
Laminating the yoke structure with a non-magnetic insulative material, such as is disclosed U.S. Pat. No. 5,750,275, by Katz et al., entitled THIN FILM HEADS WITH INSULATED LAMINATION FOR IMPROVED HIGH FREQUENCY PERFORMANCE, issued May 12, 1998, herein incorporated by reference in its entirety, improves high frequency performance over conventional single layer structures. The insulative material reduces eddy currents in the structure. Nevertheless, laminating with an insulative material also causes a reduction in the magnetic moment. Another drawback with this structure is that the non-magnetic laminating layers must be stopped short of the air bearing surface to allow flux to travel toward the write gap within the pole tip.
To improve the permeability and flux rise time at high frequency, the yoke and pole material are often formed with a low but non-zero induced uniaxial anisotropy. The induced uniaxial anisotropy controls the magnetic domain pattern of the material. A hard-axis state, one where the magnetization domains are oriented perpendicular to the flux path, is necessary to ensure that the magnetization change is conducted via rotation. This maximizes the high frequency permeability.
The uniaxial anisotropy may be induced by applying a magnetic field, or by applying an anisotropic stress to the material during deposition. In current applications, an RF sputtering process is often employed to deposit high moment materials during which an aligning magnetic field is also applied. Such RF sputtering techniques, however, are not practical for multilayer laminated devices. RF sputtering techniques, have low throughput so provide a significant impediment to commercial viability of multilayer laminated devices. A DC magnetron sputtering process, which is superior in producing high quality films with high throughput, often results in the reduction in the effectiveness of aligning field, consequently, the deposited films have poorly defined anisotropy direction.
When dispersion in the local anisotropy axes directions is significant in the as-deposited films, annealing in a magnetic field is often used. The annealing conditions which are needed to induce well-defined anisotropy in the writer, however, often causes the loss of antiferromagnetic coupling as well as undesirable interdiffusion in the reader part of the GMR type devices.